The invention relates to coal gasifiers and in particular to entrained flow slagging type gasifiers.
Entrained flow coal gasifiers which are illustrated in U.S. Pat. No. 4,343,627 issued to Michael C. Tanca on Aug. 10, 1982 operate by injecting a carbonaceous material (including recycled char) into the lower portion of the gasifier along with an air supply. Combustion in this portion of the gasifier supplies heat for the gasification reaction and should be maintained at such a temperature level as to permit the liquid ash or slag to flow out through a slag tap opening at the bottom. This portion of the gasifier is generally termed the combustion zone or combustor, and is operated at near stoichiometric conditions to obtain the maximum heat and maximum temperature to facilitate the removal of the slag.
These combustion products are conveyed upwardly to a reduction zone, or reductor where additional fuel in the form of carbonaceous material including pulverized coal is added. This additional fuel is devolatized with the remaining carbon residue, termed char, reacting with the combustion products of the reduction zone to form a combustible gas which is largely carbon monoxide. The gasification reaction is an endothermic reaction obtaining its heat from the combustion products formed in the combustion zone. This gasification process continues until the temperature level of about 1700.degree. F. is reached at which the gasification rate is too slow for practical purposes. Any remaining char particles are removed and recycled either to the combustion zone or the reduction zone.
The maximum obtainable temperature is desired in the combustion zone for the purpose of facilitating combustion of the solid carbon particles and for maintaining the slag as fluid as possible so that it will flow freely. Radiation from the combustion zone to the reduction zone tends to decrease the temperature in the combustion zone. Accordingly, it has been known to neck down the gasifier between the combustion zone and reduction zone by bending the walls of the gasifier inwardly to form a relatively narrow opening, in the order of 50 percent of the overall flow area of the gasifier. This construction leads to very expensive fabrication problems in bending and spacing the tubes as well as finning the spacing as it varies along the length of the tubes. It furthermore leads to several potential operating problems.
With centrifugal action within the gasifier caused by tangential injection of the fuel a low pressure core exists. There is a tendency for gas from the reduction zone to be drawn down through this core into the combustion zone section. This not only causes an unwanted cooling of the combustor but also draws down the desired end product, combustible gas, where it may be burned in preference to the carbonaceous material which is intended to be burned in the combustion zone.
Slag forming in the reduction section passes down along the walls, and with the centrally located opening drops to a location extremely close to the slag tap opening. Because of the lower temperature in the reductor the slag is relatively cool and accordingly does not flow freely. Dropping of this slag near the edge of the slag tap opening therefore tends to promote plugging of the opening.
It is accordingly desirable to have the two stage gasifier where radiation between the combustor and reductor is minimized, wherein gas is not drawn back from the reductor into the combustor, and where slag dropping from the reductor will fall to an outboard location within the combustor so that it has time to be heated and flow freely before it reaches the slag tap opening.